An optical waveguide mixes and directs light emitted by one or more light sources, such as one or more light emitting diodes (LEDs). A typical optical waveguide includes three main components: one or more coupling surfaces or elements, one or more distribution elements, and one or more extraction elements. The coupling component(s) direct light into the distribution element(s), and condition the light to interact with the subsequent components. The one or more distribution elements control how light flows through the waveguide and is dependent on the waveguide geometry and material. The extraction element(s) determine how light is removed by controlling where and in what direction the light exits the waveguide.
When designing a coupling element, the primary considerations are: maximizing the efficiency of light transfer from the source into the waveguide; controlling the location of light injected into the waveguide; and controlling the angular distribution of the light in the waveguide. The coupling element of a waveguide may be comprised of one or more of a number of optical elements, including a ‘primary’ source optic (such as the lens on an LED component package), one or more intermediate optical elements (such as a lens or array of lenses) interposed between the source and the waveguide coupling surface or surfaces, one or more reflective or scattering surfaces surrounding the sources, and specific optical geometries formed in the waveguide coupling surfaces themselves. Proper design of the elements that comprise the coupling element can provide control over the spatial and angular spread of light within the waveguide (and thus how the light interacts with the extraction elements), maximize the coupling efficiency of light into the waveguide, and improve the mixing of light from various sources within the waveguide (which is particularly important when the color from the sources varies—either by design or due to normal bin-to-bin variation in lighting components). The elements of the waveguide coupling system can use refraction, reflection, total internal reflection, and surface or volume scattering to control the distribution of light injected into the waveguide.
To increase the coupling of light from a light source into a waveguide, it is desirable to maximize the number of light rays emitted by the source(s) that impinge directly upon the waveguide coupling surface. Light rays that are not directly incident on the waveguide from the source must undergo one or more reflections or scattering events prior to reaching the waveguide coupling surface. Each such ray is subject to absorption at each reflection or scattering event, leading to light loss and inefficiencies. Further, each ray that is incident on the coupling surface has a portion that is reflected (Fresnel reflection) and a portion that is transmitted into the waveguide. The percentage that is reflected is smallest when the ray strikes the coupling surface at an angle of incidence relative to the surface normal close to zero (i.e., approximately normal to the surface). The percentage that is reflected is largest when the ray is incident at a large angle relative to the surface normal of the coupling surface (i.e., approximately parallel to the surface). To increase efficiency, the coupling of the light into the waveguide body minimizes the absorbing of light at reflection or scattering events as well as the Fresnel reflection at the coupling surface.
In conventional coupling, a light source, typically emitting a Lambertian distribution of light, is positioned adjacent to the edge of a planar waveguide element. The amount of light that directly strikes the coupling surface of the waveguide in this case is limited due to the wide angular distribution of the source and the relatively small solid angle represented by the adjacent planar surface. To increase the amount of light that directly strikes the coupling surface, a “flat package” component such as the Cree ML-series or MK-series (manufactured and sold by Cree, Inc. of Durham, N.C., the assignee of the present application) may be used. A flat package component is a light source that does not include a primary optic or lens formed about an LED chip. The flat emitting surface of the flat package component may be placed in close proximity to the coupling surface of the waveguide. While this arrangement helps ensure a large portion of the emitted light is directly incident on the waveguide, overall system efficiency generally suffers as flat package components are typically less efficient than components having primary lenses, which facilitate light extraction from the component, improving overall efficiency.
As discussed above, the use of higher-efficiency LEDs having conventional (e.g., predominantly hemispherical or cubic) primary optics results in a limited amount of light that is directly incident on the coupling surface of the waveguide. Such light source(s) are often placed in a reflective channel or cavity to reflect light onto the coupling surface, thereby increasing the amount of light from the source that reaches the waveguide but also reducing overall system efficiency due to the loss incurred at each reflection event. In some luminaires, the waveguide(s) may have coupling surfaces specifically shaped to maximize the amount of light captured at the coupling surfaces. For example, copending application International Application No. PCT/US14/13937, filed Jan. 30, 2014, entitled “Optical Waveguide Bodies and Luminaires Utilizing Same”, owned by the assignee of the present application, discloses a coupling cavity design comprising a plurality of ridges and grooves. However, such features can add complexity to the waveguide design and cost to the resulting waveguide-based luminaire.
Alternatively, each LED may be positioned in a cylindrical coupling cavity within the waveguide, and a reflective cap having a cone-shaped plug diverter may be placed at the opposite end of the coupling cavity, as described in copending U.S. patent application Ser. No. 13/839,949, filed Mar. 15, 2013, entitled “Optical Waveguide and Lamp Including Same,” U.S. patent application Ser. No. 14/101,086, filed Dec. 9, 2013, entitled “Optical Waveguides and Luminaires Incorporating Same,” U.S. patent application Ser. No. 14/101,132, filed Dec. 9, 2013, entitled “Waveguide Bodies Including Redirection Features and Methods of Producing Same,” and U.S. patent application Ser. No. 14/101,147, filed Dec. 9, 2013, entitled “Luminaires Using Waveguide Bodies and Optical Elements.” This type of coupling configuration can greatly increase the portion of light emitted by the source that is directly incident on the waveguide coupling surface, leading to improved coupling efficiency. However, by its nature such coupling requires discrete sources spaced remotely across a waveguide. Such discrete source placement can have advantages for thermal management of heat generated by the LED sources, but can also lead to increased cost compared to arrangements where the LED sources are all affixed to a single printed circuit board. Additionally, steps must be taken to prevent inadequate color mixing that would otherwise lead to non-uniform appearance in the luminance of the waveguide.
After light has been coupled into the waveguide, it must be guided and conditioned to the locations of extraction. The simplest example is a fiber-optic cable, which is designed to transport light from one end of the cable to another with minimal loss in between. To achieve this, fiber optic cables are only gradually curved and sharp bends in the waveguide are avoided. In accordance with well-known principles of total internal reflection light traveling through a waveguide is reflected back into the waveguide from an outer surface thereof, provided that the incident light does not exceed a critical angle with respect to the surface. Specifically, the light rays continue to travel through the waveguide until such rays strike an index interface surface at a particular angle less than an angle measured with respect to a line normal to the surface point at which the light rays are incident (or, equivalently, until the light rays exceed an angle measured with respect to a line tangent to the surface point at which the light rays are incident) and the light rays escape.
In order for an extraction element to remove light from the waveguide, the light must first contact the feature comprising the element. By appropriately shaping the waveguide surfaces, one can control the flow of light across the extraction feature(s) and thus influence both the position from which light is emitted and the angular distribution of the emitted light. Specifically, the design of the coupling and distribution surfaces, in combination with the spacing (distribution), shape, and other characteristic(s) of the extraction features provides control over the appearance of the waveguide (luminance), its resulting light distribution (illuminance), and system optical efficiency.
Hulse U.S. Pat. No. 5,812,714 discloses a waveguide bend element configured to change a direction of travel of light from a first direction to a second direction. The waveguide bend element includes a collector element that collects light emitted from a light source and directs the light into an input face of the waveguide bend element. Light entering the bend element is reflected internally along an outer surface and exits the element at an output face. The outer surface comprises beveled angular surfaces or a curved surface oriented such that most of the light entering the bend element is internally reflected until the light reaches the output face
Parker et al. U.S. Pat. No. 5,613,751 discloses a light emitting panel assembly that comprises a transparent light emitting panel having a light input surface, a light transition area, and one or more light sources. Light sources are preferably embedded or bonded in the light transition area to eliminate any air gaps, thus reducing light loss and maximizing the emitted light. The light transition area may include reflective and/or refractive surfaces around and behind each light source to reflect and/or refract and focus the light more efficiently through the light transition area into the light input surface of the light-emitting panel. A pattern of light extracting deformities, or any change in the shape or geometry of the panel surface, and/or coating that causes a portion of the light to be emitted, may be provided on one or both sides of the panel members. A variable pattern of deformities may break up the light rays such that the internal angle of reflection of a portion of the light rays will be great enough to cause the light rays either to be emitted out of the panel or reflected back through the panel and emitted out of the other side.
Shipman, U.S. Pat. No. 3,532,871 discloses a combination running light reflector having two light sources, each of which, when illuminated, develops light that is directed onto a polished surface of a projection. The light is reflected onto a cone-shaped reflector. The light is transversely reflected into a main body and impinges on prisms that direct the light out of the main body.
Simon U.S. Pat. No. 5,897,201 discloses various embodiments of architectural lighting that is distributed from contained radially collimated light. A quasi-point source develops light that is collimated in a radially outward direction and exit means of distribution optics direct the collimated light out of the optics.
Kelly et al. U.S. Pat. No. 8,430,548 discloses light fixtures that use a variety of light sources, such as an incandescent bulb, a fluorescent tube and multiple LEDs. A volumetric diffuser controls the spatial luminance uniformity and angular spread of light from the light fixture. The volumetric diffuser includes one or more regions of volumetric light scattering particles. The volumetric diffuser may be used in conjunction with a waveguide to extract light.
Dau et al U.S. Pat. No. 8,506,112 discloses illumination devices having multiple light emitting elements, such as LEDs disposed in a row. A collimating optical element receives light developed by the LEDs and a light guide directs the collimated light from the optical element to an optical extractor, which extracts the light.
A.L.P. Lighting Components, Inc. of Niles, Ill., manufactures a waveguide having a wedge shape with a thick end, a narrow end, and two main faces therebetween. Pyramid-shaped extraction features are formed on both main faces. The wedge waveguide is used as an exit sign such that the thick end of the sign is positioned adjacent a ceiling and the narrow end extends downwardly. Light enters the waveguide at the thick end and is directed down and away from the waveguide by the pyramid-shaped extraction features.
In designing waveguide/coupler systems, an important consideration is overall system efficiency, as mentioned above. For example, low-profile LED-based luminaires for general lighting applications have recently been developed (e.g., General Electric's ET series panel troffers) that utilize a string of LED components directed into the edge of a waveguiding element (an “edge-lit” approach). However, such luminaires typically suffer from low efficiency due to losses inherent in coupling light emitted from a predominantly Lambertian emitting source such as a LED component into the narrow edge of a waveguide plane.
Smith U.S. Pat. Nos. 7,083,313 and 7,520,650 discloses a light direction device for use with LEDs. In one embodiment, the light direction device includes a plurality of opposing collimators disposed about a plurality of LEDs on one side of the device. Each collimator collimates light developed by the LEDs and directs the collimated light through output surfaces of the collimators toward angled reflectors disposed on a second side opposite the first side of the device. The collimated light reflects off the reflectors out of from the one side perpendicular thereto. In another embodiment, the collimators are integral with a waveguide having reflective surfaces disposed on a second side of the waveguide, and the collimated light is directed toward the reflective surfaces. The light incident on the reflective surfaces is directed from the one side of the device, as in the one embodiment.